Validating kQ =1.0 assumption in TG51 with PTW 30013 farmer chamber for Varian TrueBeam's 2.5 MV imaging beam

Abstract

AAPM Report 142 recommends and the State of Ohio requires that the imaging dose be quantified in radiotherapy applications. Using the TG51 dose calibration protocol for MV Imaging dose measurement requires knowledge of the k_Q parameter for the beam quality and the ionization chamber type under investigation. The %dd(10)_x of the Varian TrueBeam 2.5 MV imaging beam falls outside the range of the available data for the calculation of the k_Q value. Due to the similarities of the 2.5 MV imaging beam and the ⁶⁰ Co beam, we and others made the assumption that k_Q = 1.0 in TG51 calculations. In this study, we used the TG21 and TG51 calibration protocols in conjunction to validate that k_Q = 1.0 for the 2.5 MV imaging beam using a PTW 30013 farmer chamber. Standard measurements for TG51 absolute dosimetry QA were performed at 100 cm SSD, 10 cm depth, 10 × 10 field size, delivering 100 Monitor Units to a waterproof Farmer Chamber (PTW TN30013) for both 2.5 and 6 MV. Both the TG21 and TG51 formalisms were used to calculate the dose to water per MU at d_max (D_w /MU) for the 6 MV beam. The calculated outputs were 1.0005 and 1.0004 cGy/MU respectively. The TG21 formalism was then used to calculate (D_w /MU) for the 2.5 MV imaging beam. This value was then used in the TG51 formalism to find k_Q for the 2.5 MV imaging beam. A k_Q value of 1.00 ± 0.01 was calculated for 2.5 MV using this method.

Keywords: 2.5 MV imaging dose; dose calibration.

Figure 1

Measured PDD for the 2.5 MV imaging beam after shifting to the effective point of measurement.